The present invention relates to a signal processing apparatus for converting analog signals to digital signals and processing the resultant digital signals, and the present invention is suitable for an integrated circuit for conducting processing at high speed. More particularly, the present invention relates to a circuit configuration in applying the above described signal processing apparatus as an adaptive equalizer, and the present invention is suitable for magnetic recording/reproducing apparatuses.
In various data collection systems for converting input analog signals to digital signals and conducting product summing operation and signal processing apparatuses for conducting digital processing on output signals of analog sensors, a combination of an A/D (Analog/Digital) converter and a signal processing circuit has heretofore been used.
Each A/D converter compares an input signal with a reference signal to obtain a binary value, converts the input signal to a binary number representation, and outputs the binary number representation. In many A/D converters, a thermometer code is generated according to the magnitude of an analog input signal and a change point thereof is detected. The signal of this detected change point is encoded, converted to a binary code, and then output.
In digital signal processing circuits, processing is conducted in various ways. In many cases, however, filtering for equalizing the input signal to a specific waveform is conducted at the beginning. Filtering is processing of multiplying the input signal by a plurality of filter coefficients, providing resultant products respectively with predetermined delays, and adding up the products thus delayed. It is known that a configuration in which a look-up table including a memory circuit is used for multiplication of filter coefficients is effective to reduce power consumption for processing in which filter coefficients do not change every moment. An example of a digital filter formed by using a look-up table is described in IEEE Proceedings of ISSCC '90, TAM 7.3. The look-up table used here stores the product of a memory address value and a predetermined coefficient at each address and outputs the product of data and a predetermined coefficient when an address associated with data has been selected.
As described before, the A/D converter first converts an analog input signal to a thermometer code. In the thermometer code, all bits as far as the bit position associated with the signal level have high levels and all bits above the bit position have low levels. Then a signal indicating the change point of this thermometer code is generated. By using the signal as an input selection signal, a binary encoder is driven to obtain a digital output. Meanwhile, in the look-up table, a digital signal of a binary number is decoded to generate a word selection signal. The word selection signal is a memory address selection signal for selecting a plurality of bits as a word.
If a signal processing circuit provided with an analog signal as its input is formed by simply connecting a conventional A/D converter as described above with a conventional look-up table as described above, then the circuit scale becomes large, and power consumption increases wastefully. It has also been found that in this case the processing speed cannot be made sufficiently high. Especially if an integrated circuit having functions of both circuits is to be formed, reduction of both the circuit scale and power consumption become an important problem.